1. Field of the Invention
The invention relates to a planet roller speed changer.
2. Description of the Related Art
A planet roller speed changer (traction drive) is conventionally known. The planet roller speed changer includes an input shaft, an output shaft, a stationary ring, a plurality of cylindrical planet rollers, and a carrier fixed to the output shaft. The stationary ring is disposed concentrically with the input shaft. The planet rollers are brought into rolling contact with both the input shaft and the stationary ring. The carrier is fixed to the output shaft. There is a planet roller speed changer in which pins that protrude from a carrier are passed through the inner peripheries of planet rollers such that the planet rollers are rotatably supported.
If there is a clearance between the inner periphery of each planet roller and the outer periphery of a corresponding one of the pins, the clearance serves as a backlash and may cause rotational fluctuations during revolution of the planet rollers. In order to prevent such rotational fluctuations, for example, Japanese Patent Application Publication No. 6-174026 (JP 6-174026 A) proposes that a double-row radial bearing including two radial bearings, such as a double-row deep groove ball bearing, be disposed between the inner periphery of each planet roller and the outer periphery of a corresponding one of the pins.
In order to further effectively prevent formation of the above-described backlash in the planet roller speed changer as described in JP 6-174026 A, a preload may be applied to each of the radial bearings respectively disposed in two rows, thereby maintaining the bearing internal clearance of each of the radial bearings at a negative clearance. For example, a positioning preload may be adopted as a preload applied to each of the radial bearings. For example, when the first and second radial bearings are angular contact bearings, it is necessary to control the stand out of each radial bearing in order to maintain a thrust internal clearance at a negative clearance.
However, an appropriate magnitude of preload needs to be applied to the radial bearings described above. If the magnitude of preload applied to each of the radial bearings is too low, a clearance may be formed between the inner periphery of each of the planet rollers and the outer periphery of a corresponding one of the radial bearings. On the other hand, if the magnitude of preload applied to each of the radial bearings is too high, smooth revolution of the planet rollers may be hindered or indentations may be formed in the inner peripheries of the planet rollers. Further, in the planet roller speed changer, each of the planet rollers is disposed so as to be pressed against both the stationary ring and the input shaft for torque transmission, the inner diameter of each planet roller varies in the circumferential direction. With this variation in inner diameter taken into account, it is necessary to apply a preload having a magnitude within such a range that no clearance is left between the planet roller and the radial bearing and no indentations or the like are formed. For this reason, radial bearings interposed between the planet rollers and the pins are required to have an extremely high accuracy of the bearing internal clearance. This may result in a cost increase.